Studies are being carried out on the actions of both diets and drugs used in treatment of hyperlipidemia. Our results show that polyunsaturated fats cause a reduction of very low density lipoproteins in patients with hypertriglyceridemia, and this decrease is associated with mobilization of cholesterol from the body. In the attempt to maximize plasma lipid lowering, several drugs have been employed in combination. A marked reduction in plasma lipids can be obtained by the combined use of clofibrate and Colestipol, but this combination is associated with an increased saturation of bile with cholesterol and therefore it may be accompanied with a transitory increase in the risk for gallstone formation. Likewise, significant lipid lowering can be achieved in many hyperlipidemic patients with obesity, but again, weight reduction may be associated with increased cholelithiasis. Thus, cholesterol gallstones may be an important side effect resulting from a variety of therapeutic approaches to the lowering of plasma lipids.